An example of a construction vehicle is one wherein a hydraulic pump is driven by an engine, and a travel hydraulic motor is driven by pressure oil discharged from the hydraulic pump, thereby causing the vehicle to travel. With this type of construction vehicle, the speed and traction force of the vehicle can be controlled by controlling the engine speed, the displacement of the hydraulic pump, and the displacement of the travel hydraulic motor (Japanese Laid-Open Patent Application No. 2004-144254).
Pump displacement-primary circuit oil pressure characteristics such as those shown in FIG. 14 are commonly obtained in a construction vehicle such as the one described above. The solid line L11 and the dashed lines L12 to L15 in this diagram are lines showing the pump displacement-primary circuit oil pressure characteristics, which vary according to the engine speed. The primary circuit oil pressure is the oil pressure in a travel hydraulic circuit through which flows pressure oil discharged from the hydraulic pump and sent to the travel hydraulic motor. The dashed line L14 shows that the pump displacement is reduced when the primary circuit oil pressure increases, and the pump displacement is increased when the primary circuit oil pressure decreases. The construction vehicle is also provided with an oil pressure limiting component in order to protect the travel hydraulic circuit. The oil pressure limiting component is configured from a cutoff valve or another component for reducing the primary circuit oil pressure so that the primary circuit oil pressure does not exceed a predetermined cutoff pressure value. In the pump displacement-primary circuit oil pressure characteristics shown by the dashed line L12 in FIG. 14, the cutoff valve operates in cases in which the pump displacement is equal to or less than a predetermined pump displacement value Qx3, causing the primary circuit oil pressure to decrease and remain constant at a cutoff pressure value Plt.